1. Field of the Invention
The present invention relates to a system for detecting a sound pressure by means of using a detecting assembly composed of a piezoelectric substrate and input- and output interdigital transducers.
2. Description of the Prior Art
Stethoscope is a typical one of conventional devices for detecting a sound pressure. When using a stethoscope, a sound pressure based on a heartbeat comes to one""s ears directly, so that not only to have high-sensitive ears but also to be rich in experience are essential to check the heartbeat. It is difficult to remain a sound in the form of a document from an objective viewpoint, whether the sound is to be heard or not to be heard by one""s ears. Moreover, it is impossible for stethoscope to transduce a sound pressure to an electric signal.
In general, it is under consideration to utilize a hydrophone for transducing a sound pressure to an electric signal, the hydrophone with a bulk wave type of piezoelectric vibrator operating in thickness mode. The hydrophone has some problems on sensitivity, limitation of operation frequencies, difficulty in use, resistance for a change of circumstances, resistance for noises, and so on. Thus, the hydrophone operating in thickness mode has difficulty in transducing a sound pressure to an electric signal.
An object of the present invention is to provide a sound pressure detecting system capable of sensing a sound with a high sensitivity, whether the sound is to be heard or not to be heard by one""s ears.
Another object of the present invention is to provide a sound pressure (detecting system capable of operating in ultrasound.
Another object of the present invention is to provide a sound pressure detecting system capable of transducing a sound pressure to an electric signal and displaying the sound pressure on a monitor.
Another object of the present invention is to provide a sound pressure detecting system excellent in measurement accuracy, response time, durability, manufacturing, resistance for a change of circumstances, and resistance for noises.
A still other object of the present invention is to provide a sound pressure detecting system easy in use and having a small size which is very light in weight and has a simple structure.
According to one aspect of the present invention there is provided a sound pressure detecting system comprising a piezoelectric substrate, input- and output interdigital transducers formed on an upper end surface of the piezoelectric substrate, a liquid cavity, an air cavity, a partition wall between the liquid cavity and the air cavity, a signal analyzing unit connected between the input- and output interdigital transducers, and a monitor connected with the signal analyzing unit.
When an input electric signal is applied to the input interdigital transducer, a leaky elastic wave is excited in the piezoelectric substrate. The leaky elastic wave is radiated effectively in the form of a longitudinal wave into a liquid in the liquid cavity, the liquid being in contact with a lower end surface of the piezoelectric substrate. The partition wall reflects the longitudinal wave in the liquid. A reflected longitudinal wave is detected at the output interdigital transducer as a delayed electric signal. A sound pressure through the partition wall from the outside can be sensed by the signal analyzing unit with a high sensitivity in terms of a change in the delayed electric signal, and then, displayed on the monitor.
According to another aspect of the present invention there is provided a piezoelectric substrate made of a piezoelectric ceramic, the polarization axis thereof being parallel to the thickness direction thereof
According to another aspect of the present invention there is provided a piezoelectric substrate made of a piezoelectric polymer film.
According to another aspect of the present invention there is provided a signal analyzing unit comprising a signal generator and a phase comparator. The signal generator generates the input electric signal. The phase comparator compares a phase of the input electric signal with that of the delayed electric signal, and senses the sound pressure in terms of a difference between the phase of the input electric signal and that of the delayed electric signal.
According to other aspect of the present invention there is provided a signal analyzing unit comprising an amplifier and a frequency counter. The amplifier amplifies the delayed electric signal, and delivers an amplified electric signal. The input interdigital transducer receives a part of the amplified electric signal as the input electric signal. Thus, the input- and output interdigital transducers and the amplifier form a delay-line oscillator. On the other hand, the frequency counter counts a frequency of the amplified electric signal, and senses the sound pressure in terms of a change in frequency of the amplified electric signal.
According to a further aspect of the present invention there is provided a signal analyzing unit comprising an amplifier and a frequency to voltage (F/V) converter. The amplifier amplifies the delayed electric signal, and delivers an amplified electric signal. The input interdigital transducer receives a part of the amplified electric signal as the input electric signal. Thus, the input- and output interdigital transducers and the amplifier form a delay-line oscillator. On the other hand, the F/V converter converts a frequency of the amplified electric signal into a voltage thereof, and senses the sound pressure in terms of a change in voltage of the F/V converted electric signal.